This invention relates generally as indicated to a centrifugal pump especially suited for use in pumping aircraft fuels and the like, and more particularly to a pump employing a variable speed centrifugal impeller stage in combination with a positive displacement pump device for matching fuel flow and pressure to engine requirements throughout a variable flight profile.
In modern aircraft fuel systems, fuel is generally supplied to the engine of the aircraft by an engine-driven fuel pump. Because engine fuel requirements vary under flight conditions, such fuel systems normally require a high flow turn-down ratio of approximately 50:1 for engines without afterburners and 150:1 for engines with afterburners. At flight idle and at high altitudes and high engine speeds, for example, engine fuel flow requirements are relatively low. Without sufficient flow turn-down under such conditions, high input speed to the pump being driven by the engine combined with low fuel flow can cause excessive fuel temperature rise. Since the varying environment in which the aircraft operates may cause the fuel to vary in temperature over a wide range for example from -65.degree. to +135.degree. F. or more, and the temperature rise due to insufficient turn-down may exceed 100.degree. F., the temperature of the fuel being supplied to the engine may exceed 320.degree. F. at which temperature gas turbine burner nozzle coking is known to occur.
Heretofore, various pump designs have employed an engine driven, positive displacement pump within the contaminated fuel region with or without a centrifugal impeller at its first stage. These pumps, however, employ close clearance displacement members that are subject to rapid wear or catastrophic failure with high fuel contamination. To prevent pump failure, usually a filter is provided in the fuel line; however, such filter requires routine service and itself is subject to clogging leading to insufficient fuel flow and thus degraded engine performance. Such pumps further require hydromechanical in-line fuel controls for obtaining high flow turn-down to prevent development of high fuel temperatures. Nevertheless, the fuel temperature delivered to the gas turbine burner nozzle by such type of pump may be sufficiently high to cause coking unless cooled for example by utilizing turbine bleed air but with consequent reduction in gas turbine efficiency.
Centrifugal pumps alone or in combination with jet pumps have also been used in aircraft fuel systems. An example of one such centrifugal pump capable of handling a high ratio of fuel vapor and/or air and liquid fue, termed V/L, greater than 1.0 is shown and described in Davis et al U.S. Pat. No. 4,142,839, granted Mar. 6, 1979. In such centrifugal pumps, fuel discharge output is proportional to input speed resulting in high flow and pressure at high engine speeds. However, under low flow conditions which, for example, occur at high altitudes and high engine speeds, excess pressure may cause high fuel temperatures in excess of 320.degree. F. thereby leading to possible gas turbine burner nozzle coking. Accordingly, suitable in-line fuel controls and metering devices are required to regulate the fuel discharge, and then feed such fuel to a fuel metering device for controlled flow to the fuel nozzles. Moreover, centrifugal pumps by themselves generally cannot develop at low RPM's sufficient fuel flow and pressure to provide for pump prime and engine start-up.